In recent years, a steering device of a system called steer by wire (SBW) is being developed instead of a steering device of mechanical steering system. In the mechanical steering system, a steering wheel is mechanically coupled with tires. Owing to this, a vibration or a swing due to irregularities of a road surface may be transmitted to the steering wheel. To correct the influence of the vibration or the swing, a driver unconsciously operates the steering wheel. This may cause stress or tiredness during driving.
In contrast, in the steer by wire system, a steering wheel is not mechanically coupled with tires, and steering by the steering wheel and change of a turning angle of the tires are individually controlled. A steering device detects steering by the steering wheel with a sensor, and transmits a control signal calculated based on sensor information to a turning motor that controls the turning angle of the tires through a wire harness. The turning motor turns the tires by driving a turning mechanism. With the steering device of the steer by wire system, a vibration or a swing due to irregularities of a road surface is not transmitted to the steering wheel. Accordingly, stress or tiredness during driving is reduced.
In preparation for occurrence of an error in the control signal of the steer by wire system, in many cases, the steering device may switch a system between the steer by wire system and the mechanical steering system. If an error occurs in the steer by wire system, the steering device causes the steering wheel to be mechanically coupled with the turning mechanism by a clutch mechanism, switches the system to the mechanical steering system, and hence allows a steering operation to be executed.
The steering device of the steer by wire system has a problem in which the viscosity of grease in a turning gear box increases and the friction of the turning gear box increases at low temperatures.
Also, there is suggested a technology described in Japanese Unexamined Patent Application Publication No. 2010-132036 (hereinafter, referred to as document '036). Abstract of the document '036 describes a configuration that “in a turning device 12 in which a nut 44 formed with a female screw is thread-engaged with a male screw formed on a turning rod 32 and a rotation force is given to the nut by a motor, a material having a larger linear expansion coefficient than an inner race 52 of a bearing 50 for retaining the nut and the steering rod is used for the nut, and a pressure-fitting margin of the nut to the inner race and a clearance of the male screw and the female screw are adjusted such that the pressure-fitting margin does not become 0 and the clearance does not become 0 even at the lowest use allowance temperature.” Also, Abstract of the document '036 describes an advantageous effect that “a contraction amount difference of an outer diameter of the nut and an inner diameter of the inner race by temperature variation and a contraction amount difference of the male screw and the female screw are compensated to absorb variation of the clearance, and even at the lowest use allowance temperature, therefore, the nut is firmly retained to the inner race and smooth relative action of the nut and the steering rod is ensured.”
However, a variation in dimension of parts forming the turning gear box has to be considered. It is difficult to provide stable quality for all products.
Regarding the steering device with the steer by wire system of related art, if the viscosity of the grease in the turning gear box increases and the friction increases at low temperatures, a deviation between a target turning angle and an actual turning angle increases, and hence control performance may decrease. Also, if the friction of the turning gear box becomes a value of a turning torque of the turning motor or higher, turning itself may be obstructed.
In this case, the steering device causes the clutch mechanism to mechanically couple the steering wheel with the turning mechanism, and the mode is shifted to, for example, an electronic power steering (EPS) mode or a manual steering mode to allow turning. However, if the mode is changed such that activation is executed with the SBW mode, and then the mode is shifted to the EPS mode immediately, the change may give the driver uncomfortable feeling.